Inherently, heat is generated by losses in the windings and the stator core of an electrical machine such as a motor or generator. These losses are due to electrical resistance in the windings and losses in the iron. These losses take the form of thermal energy. This thermal energy must be removed by thermal conduction out of the stator core. An example of an electrical machine is a permanent magnet electrical machine which takes the form of a stator core held in a frame with inwardly facing windings which interact in operation with a rotor core which normally carries permanent magnets. Thus, with electrical current sequentially passed through the windings, the rotor can be driven and turned. Alternatively, if the rotor is driven by other means then electrical current is generated in the windings.
Alternating magnetic flux in the core causes iron losses (eddy currents and hysteresis) which causes heating. Thus, the core is made from low-loss magnetic material such as silicon-iron, which reduces the eddy currents (due to low electrical conductivity) and also reduces the hysteresis loss. Inevitably low electrical conductivity means low thermal conductivity which thus inhibits cooling.
In small and medium-sized permanent magnet (PM) machines, cooling is normally achieved by heat transfer at the airgap or at the stator outside diameter (in some cases cooling ducts may also be used towards the outside of the laminated stator core). Certain high power density machines may instead have sleeved liquid cooling passages at the stator outer diameter (OD) or in the stator housing. Effective radial conduction of heat towards the stator outer diameter (OD) and towards the airgap is therefore essential.
FIG. 1 is a schematic illustration of a cross-section of a part of an electrical machine with a permanent magnet rotor. FIG. 1 is provided simply to illustrate positional relationships for better understanding of the present invention. Thus, the electrical machine 1 has a stator core 2 located within a stator housing 3. The rotor 4 is located in the middle of the stator. The rotor 4 presents permanent magnets 5 to windings 6 and teeth 14 of the stator core 2. The windings 6 are located in stator slots 19 formed between the stator teeth 14 which protrude inwards from the core 2. These magnets 5 and windings 6 interact as described above to either drive rotor 4 motion or convert that rotor 4 motion caused by other means into electrical power. Some air cooling passages 7 are shown near the outer rim 15 of the stator core 2. In any event, heat energy generated by losses in the core 2 and the windings 6 and teeth 14 must be conducted through the core 2 to these passages 7 and/or to indirect air cooling vents 8 on the outer peripheral surface/rim of the core 2.
As indicated above the core 2 is made from materials which have been formulated for low iron losses (eddy currents and hysteresis) to minimize electrical eddy current losses in the core 2. Unfortunately such materials have lower thermal conductivity properties than is desired for radial heat conduction to the cooling surfaces.
In the above circumstances, heat energy must be conducted radially outwards from the windings 6 through the teeth 14 and core 2 towards the cooling passages 7. However, their relatively low level of thermal conductivity means that there are significant transient and steady-state temperature differentials between the windings 6 and the outside diameter of the core 2. These differentials are detrimental to operational efficiency and/or may cause premature failure of electrical insulation within the electrical machine.
The winding electrical insulation (typically polyester or polyimide) has limited temperature capability and it is the winding that always gets the hottest with consequent implications for the power rating of the electrical machine.